1. Field of the Invention
The invention relates to underwater acoustic projectors and is directed more particularly to a projector of the type commonly referred to as a "push-pull" acoustic projector.
2. Description of the Prior Art
Acoustic projectors typically are used to produce high-power low-frequency sounds in the ocean or other water body. It generally is desired to be able to generate hundreds of watts of omnidirectional acoustic power at frequencies below 1000 Hz., with a device that can be deployed from an aircraft, surface vessel, or submarine.
In order to create high-power acoustic tones in water at low frequencies, a device must be able to produce large volume displacements. The volume displacement is the integral of the normal displacement of a radiating area, taken over that area. Therefore, an acoustic projector must have a large radiating area, or a large displacement, or both. It is beneficial to have a projector with an output that varies linearly with the projector input.
The term "push-pull" as used herein and in the appended claims refers to a mode of operation of a pair of electrostrictive annular rings, in which the rings are in abutting relation to one and the other faces of a plate. The rings operate in unison, but oppositely, such that one ring "pushes" the plate, while the other ring simultaneously "pulls" the plate. Such an arrangement is commonly referred to as a "push-pull" projector, or transducer.
U.S. Pat. No. 3,725,856 to Chevenak illustrates a push-pull transducer in which a driver plate is not attached to the walls of a hollow cylinder formed by a plurality of walls.
In recent years, consideration has turned to electrostrictive rings of lead magnesium niobate (PMN) and lead magnesium niobate-lead titanate (PMNPT) ceramics. For example, U.S. Pat. No. 5,359,252 to Swift et al. discloses an actuator in which a stack of lead magnesium niobate crystals are free to expand longitudinally within a cylindrical casing to act on a piston. U.S. Pat. No. 5,493,165 to Smith et al. disclose a driver for electrostrictive actuators in which rings of lead magnesium niobate are interleaved with electrode rings to form a stack.
Lead magnesium niobate and lead magnesium niobate-lead titanate have recently gained wide interest in the underwater acoustics transduction community due to the propensity of these materials to exhibit large strains at relatively modest electric field levels. One difficulty with these materials, however, is that the observed strains exhibit a nonlinear response to the applied drive voltage. That is, harmonics of the drive frequency appear in the response of the material. However, the nonlinearity of both PMN and PMNPT is known to exhibit a primarily quadratic response of the strain to the applied drive. See K. M. Rittenmeyer, "Electrostrictive Ceramics for Underwater Transducer Applications," J. Acoust. Soc. Am. 95, pp. 849-856 (1994). This quadratic behavior has been shown to permit the application of a revised Hunt electrostatic transducer model (F. V. Hunt, Electroacoustics, John Wiley & Sons, New York, 1954, pp. 176-177) to understanding the behavior of a PMN or PMNPT-based transducer. See J. C. Piquette, "A Fully Mechanical Transducer Model With Application to Generalizing the Non-Linear Hunt Electrostatic Transducer for Harmonic and Transient Suppression," J. Acoust. Soc. Am. 98, pp. 422-430 (1995). Since PMN and PMNPT behave in a manner similar to a Hunt electrostatic transducer, concepts applicable to linearizing an electrostatic transducer are also applicable to developing a linear underwater projector which utilizes either a PMN and/or PMNPT active element. Applicant has recognized the concept of push-pull electrostatic loudspeaker is applicable to the development of a push-pull underwater projector using PMN and/or PMNPT. It also has been recognized by applicant that since the properties of PMN are sensitive to the operating environmental conditions, it is also important to correct the operation of a PMN-based projector for variations in environmental conditions.
The operation of an electrostatic loudspeaker is based on the push-pull principle. This principle produces cancellation of nonlinear responses that arise from a quadratic nonlinearity. The push-pull electrostatic loudspeaker operates in a mode which attempts to maintain a constant charge on a pair of balanced moving plate capacitors which share a common moving plate. It has been recognized by applicant that an underwater projector using a PMN and/or PMNPT active element can also be designed to take advantage of the push-pull concept, and hence can also produce a linearized output in a manner similar to an electrostatic loudspeaker.